1. Field of the Invention
The present invention relates to an X-ray computed tomography (CT) apparatus that takes an image of a subject by irradiating the subject with X-rays, collecting projection data, and reconstructing an image from the collected projection data. In particular, the present invention relates to an imaging condition setting method used when a wide area including a heart is imaged.
2. Description of the Related Art
Conventionally, an X-ray CT apparatus irradiates a subject with X-rays and collects projection data by detecting the X-rays passing through the subject. The X-ray CT apparatus then reconstructs an image from the collected projection data. In recent years, heart examinations have generally been performed using the X-ray CT apparatus. When the X-ray CT apparatus takes an image of a heart, a method is used in which imaging is performed synchronously with a cardiac cycle of the heart. The method is referred to as “electrocardiographic synchronized imaging” (refer to, for example, JP-A 2007-117719 (KOKAI)).
On the other hand, an imaging method referred to as “step-and-shoot” is also used by the X-ray CT apparatus. In “step-and-shoot”, a portion of the subject is irradiated with the X-rays and the irradiated portion is then imaged. Subsequently, while irradiation with the X-rays is stopped, the subject and a top plate on which the subject is placed are moved in a body axis direction by a predetermined distance. Another portion of the subject is irradiated with the X-rays and the irradiated portion is then imaged. Movement and imaging are repeated, allowing a wide area of the subject to be imaged.
When a wide area including a heart is imaged using the X-ray CT apparatus, imaging is performed by a combination of electrocardiographic synchronized imaging and “step-and-shoot”.
In the above-described electrocardiographic synchronized imaging, imaging is timed to be synchronous with the cardiac cycle. Therefore, wait times occur in correspondence with the cardiac cycle. In actuality, imaging is performed every one to two heartbeats. Therefore, electrocardiographic synchronized imaging is time-consuming compared to ordinary imaging (imaging that is not synchronized with the cardiac cycle).
The heart is an organ that constantly repeats contraction and expansion with each heartbeat. However, for example, an examination of a fine organ, such as a coronary artery, requires a clear image that is little affected by movement. To obtain an image that is little affected by movement, improvement is required in a temporal resolution of the image reconstructed from the projection data. Reconstruction methods that improve the temporal resolution are, for example, half reconstruction and segment reconstruction. In a half reconstruction operation, the image is reconstructed using projection data collected while an X-ray tube is rotating within a range of 180 degrees plus α (α being a fan angle). Compared to when the image is reconstructed using 360-degree range projection data (full reconstruction), the half reconstruction can shorten the temporal resolution by approximately one-half.
On the other hand, in a segment reconstruction operation, pieces of projection data of a same cross-section and a same phase are extracted from pieces of projection data of a predetermined number of heartbeats. The extracted pieces of projection data are combined to form a piece of projection data of a range of 180 degrees plus α. Subsequently, the half reconstruction operation is performed. Compared to when the image is reconstructed using the 360 degree range projection data, the segment reconstruction can shorten the temporal resolution to about (180+α)/n when an n number of heartbeats are used.
In this way, segment reconstruction can further improve the temporal resolution of the image, compared to half reconstruction. However, because the pieces of projection data of a plurality of heartbeats are required, imaging in segment reconstruction mode is more time-consuming than that in half reconstruction mode. Therefore, an extremely long imaging time is conventionally required to image the wide area including the heart by the combination of electrocardiographic synchronized imaging and “step-and-shoot”, and to obtain an image with high temporal resolution.
However, even when the wide area including the heart is imaged, in actuality, some regions are not required to be imaged by electrocardiographic synchronized imaging while other regions do not require an image with high temporal resolution. Specifically, a region that does not move because of being far from the heart is not required to be imaged by electrocardiographic synchronized imaging. A region that moves because of being near the heart, but has less movement than the heart, does not require an image with high temporal resolution.
Therefore, in imaging combining electrocardiographic synchronized imaging and “step-and-shoot”, optimal imaging conditions related to electrocardiographic synchronization and reconstruction modes are required to be set depending on the region to optimize the time required for a series of imaging operations. Conventionally, imaging conditions have been manually set by an operator, thus requiring a significant amount of work. Therefore, in the imaging combining the electrocardiographic synchronized imaging and “step-and-shoot”, how to automatically set the optimal imaging conditions for each region and shorten the time required for an overall imaging operation is an important issue.